The present invention relates to an antenna construction for a receiver for electromagnetic waves in a mobil communication unit adapted to be worn on the operators head, such as ear-phones, ear-muffs, helmets, spectacles etc.
When receiving directed, polarized, electromagnetic waves it is important that the receiver antenna is correctly orientated in the field which, for technical reasons, is usually entirely horizontal or vertical. Permanent receiver antennae are given the same direction as transmitting antennae in order to be able to receive the maximum value of the field strength.
If the direction of the receiver antenna deviates from that of the transmitting antenna, the voltage of the antenna drops from maximum to 0 with the cosine value of the angle. In practice a signal deviation of 3 decibels is minimal and a deviation of 10 decibels can easily be compensated by compression amplifiers. This means that a directional deviation of .+-. 30% will not be noticeable and a deviation of .+-. 60 will not disturb the receiver. However, with higher values the voltage drops rapidly to 0 and receiving becomes impossible.
Previously the inconvenience of a movable antenna with substantially portable receivers was not considered important since the apparatus were relatively large and did not invite wide angular movement. Nowadays receivers are small, often have built-in antennae and are often directly attached for example in hearing sets and the like arranged on the listener's head.
The angular movement of the head is considerable in relation to that of the body, for example, and may often be 120.degree., 90.degree. forwards and 30.degree. backwards during work (when looking at the floor or up at the ceiling). During normal work, for example at a machine or writing desk, the head is inclined about 20.degree. forward.
One object of the present invention is therefore to eliminate the above mentioned drawbacks and, in normal working positions, to avoid any fading of the receiver.
A further object of the invention is to provide a communication unit having a receiver with a new and improved antenna construction giving optimal receiving conditions for the longest possible time.
These objects, others and numerous advantages will be set forth and apparent from the following description of the invention.